
Quality of Service (QoS)
©2008 Allied Telesis Inc. All rights reserved.
Software Version 5.2.1
AlliedWare Plus
TM
Operating System Software Reference C613-50003-00 REV E
39.5
The Differential Services Field
Figure 39-3
shows an IP header containing a Differentiated Services field. The format of this
redefined field is explained in RFC 2474; the main difference being that the old ToS field has
been replaced by a 6 byte Differentiated Services Code Point (DSCP) field, which now
provides for up to 64 defined values.
By applying this model only the QoS edge routers need to fully interrogate the incoming data
packets; the QoS core routers are then relieved of this processing task and need only to inspect
the DCSP before applying its appropriate forwarding, queueing, and shaping rules.
Figure 39-3: The DSCP bits of the DS field in the IPv4 header
On the switch you can use the
match ip-dscp command on page 40.11
to select frames
containing a particular DSCP value, and associate them with a particular class map and policy
map. You can then use either the
set queue
command to directly map these incoming frames
to to one of eight egress queues. This facility enables you to accept frames that are already
carrying meaningful priority information to be automatically assigned to an appropriate egress
queue. For example, you could decide to send frames with a User Priority value of 7 to queue
3, and frames with a User Priority value of 2 to queue 7.
Because the model offers considerable flexibility, and the mapping of traffic types to DCSPs is
individual for each network, this locally applied definition is known as a
Differential Services
Domain
. The previous section introduced the concept of a Per Hop (service quality) Behaviors
or PHBs. RFC 2597 defines a specific PHB group called Assured Forwarding (AF). The AF PHB
group provides delivery of IP packets in four independently forwarded AF classes. Within each
AF class, an IP packet can be assigned one of three different levels of drop precedence.
Table 39-2
shows a list of recommended AF code points.
Table 39-2: Recommended DSCP Code Points
(Lowest Priority)
Class 1
(001xxxx)
Class 2
(010xxxx)
Class 3
(011xxxx)
(Highest Priority)
Class 4
(100xxxx)
Low Drop Precedence
001010
AF11
Decimal 10
010010
AF21
Decimal 18
011010
AF31
Decimal 26
100010
AF41
Decimal 34
Medium Drop Precedence 001100
AF12
Decimal 12
010100
AF22
Decimal 20
011100
AF32
Decimal 28
100100
AF42
Decimal 36
High Drop Precedence
001110
AF13
Decimal 14
010110
AF23
Decimal 22
011110
AF33
Decimal 30
100110
AF43
Decimal 38
DSCP value
ver IHL DiffServ
total length
identification
flags
fragment
offset
TTL
protocol
3
7
15
79
0
31
47 51
63
71
95
header
checksum
1
2 3 4
5 6
0
7
Precedence
Type of
Service
MBZ
Type of Service field
0
1
DSCP